A charged particle microscope has a still higher resolution than that of an optical microscope, and is thus widely used to clearly observe a fine structure of an observation target. In the charged particle microscope, a target sample is irradiated with a charged particle beam, particles (charged particles of the same kind as or the different kind from the applied charged particles, or electromagnetic waves or photons) which are emitted from the target sample or are transmitted through the target sample are detected with a detector, and thus an enlarged image of the target sample is acquired. A scanning charged particle microscope scans a sample with a focused charged particle beam. Examples of the microscope include a scanning electron microscope, a scanning ion microscope, and a scanning transmission electron microscope, and the microscope is used in various fields such as a material field, a semiconductor field, a food field, a biotechnology field, and a medical field.
There is an S/N ratio or a resolution as a major index related to quality of a scanning charged particle microscope image. In order to achieve a high S/N ratio or a high resolution, an electron optical system or a detection system has been continuously improved in the related art. A sufficient amount of charged particle beams are required to be applied to a sample in order to achieve a high S/N ratio in principle, but if an irradiation amount is increased, this causes a problem such as a reduction in throughput. Also in achievement of a high resolution, a resolution of the present scanning charged particle microscope comes close to a diffraction limit which is a physical restriction, and thus it is fundamentally difficult to considerably improve the resolution.
In relation to these problems, in recent years, achievement of high image quality through image processing has been examined. A signal component and noise included in an image are identified from each other, and it can be expected that a high S/N ratio is realized through a noise removal process for reducing the noise (for example, PTL 1). In addition, a deconvolution process is performed on the basis of an intensity distribution of a charged particle beam on a sample, which has the great influence on a resolution of an image, and thus the resolution can be improved (for example, PTLs 2 and 3). Further, as processes of achieving high image quality from the viewpoint different from an S/N ratio or a resolution, a method of appropriately setting contrast or reducing distortion of an image or vibration (for example, PTLs 4 and 5).
In relation to a method of capturing a charged particle microscope image, there is a high-speed scan mode in which an image is acquired by combining one or more image data items within the same visual field, obtained through high speed scanning, and a low-speed scan mode in which scanning is performed over time. In the high-speed scan mode, image blurring occurs due to vibration or the like of an applied charged particle beam. On the other hand, in the low-speed scan mode, such blurring can be prevented, but a long time such as several tens of seconds is required to capture a single image. Both of the modes are widely used, and are used depending on an application or purpose. In addition, there is an increasing demand for high image quality in both of the modes.